Work machines such as wheel loaders, and the like, include a drivetrain to drive traction devices, such as wheels, tracks, and the like, to propel the work machine over a work surface. In one typical drivetrain arrangement, a power source, such as an internal combustion engine, is operatively connected to an automatic transmission by a torque converter. The transmission is in turn operatively connected to an axle that rotates the traction devices which propel the machine over the work surface. The torque converter is a fluid coupling that allows the power source to operate at low speeds, such as when the power source is idling, by being loosely coupled to the transmission. At higher speeds developed at the output of the power source, greater fluid pressure is developed within the torque converter. When pressurized, the fluid couples the power source to the transmission. This in turn, drives the transmission and the traction devices to propel the machine. The torque converter may include a lock-up clutch that may be locked so that a power source output shaft and a transmission input shaft rotate at the same speed, and may be unlocked to allow the shafts to rotate at different speeds.
The machine may further include braking devices which apply force to the machine components, such as an axle, on which the traction devices are mounted. The braking devices apply force to the components in response to an operator of the machine. This response could be a displacement of a floor pedal depressed by the operator's foot. The brake force reduces a ground speed of the machine over the work surface. As the ground speed of the machine is reduced, the transmission may downshift to a lower gear. In these arrangements, the lock-up clutch is unlocked, if necessary, shortly after the operator input indicates a braking request. The drivetrain provides minimal retarding force on the machine during braking, due to slippage in the torque converter. As a result, the braking devices cause a majority of the retarding of the machine. Thus, the braking devices and the engaged components of the machine may experience premature wear from heat generation associated with braking of the machine.
In some other scenarios, the lock-up clutch may remain locked even after the operator input indicates a braking request. In such cases, the transmission absorbs some of the machine's translational kinetic energy, thereby assisting in the machine retardation. However, if the machine hits a pile or an obstacle with the lock-up clutch locked, a significant amount of energy may be absorbed by the transmission and its components. Absorption of a large amount of energy by the transmission components may cause a reduction of a useful life of the transmission components.
U.S. Pat. No. 6,074,326 discloses a lock-up control system for a vehicle drivetrain with a torque converter. The lock-up control system disengages a lock-up clutch based on a deceleration of a transmission shaft, and therefore a vehicle. The lock-up control system triggers the lock-up release based on a magnitude of deceleration.